Add in `file` query parameter and parser

- tested with http://localhost:8000/?file=https://api.github.com/repos/roc/deploy-lag-radiator/contents/repos/performance-platform.json?ref=gh-pages/
- gists can get lost/caught in the ether, files have better visibility
- the file parser runs atob (https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64.atob) because the github contents api passes back base64 encoded strings instead of e.g. jsonp
Merge pull request #19 from alphagov/accept_files

Add in `file` query parameter and parser